#include<iostream>
#include<iomanip>
using namespace std;
int main(){
	double x,y;
	cin>>x;
	if(x>=0&&x<5){
		y=-x+2.5;
	}
	if(x>=5&&x<10){
		y=2-(1.5*(x-3)*(x-3));
	}
	if(x>=10&&x<20){
		y=x/2-1.5;
	}
	cout<<fixed<<setprecision(3)<<y;
	return 0;
}
